Corrugated paper board or liner board is commonly prepared by first forming a corrugated element or medium by passing a cellulosic sheet between corrugating rolls forming a substantially sinusoidal or serpentine cross-section in the sheet. An adhesive is commonly applied to the tips of the thus formed sinusoidal portion called flutes and a noncorrugated or planar cellulosic liner is applied against the adhesive coated flutes of the corrugated elements as the corrugated sheet passes between a corrugating roll and a pressure roll. The resulting paper product having the corrugating medium on one side and the planar liner on another side is called a single-faced element. The single-faced element may be used as is in certain applications as a liner or buffer material within a container. More commonly adhesive is applied to the flute tips of the single-faced element and a second liner sheet is subsequently applied to the adhesive liner in a "double facer" operation. The second liner sheet is exposed to conditions of heat and pressure during its contact with the adhesive. In a so-called "hotplate" dryer section a continuously moving belt with a hotplate or platen surface is used to partially remove water from the adhesive bond line and establish a cured adhesive bond at elevated temperatures such as 300.degree.-400.degree. F.
The adhesives which have been developed to form the corrugated board or paper board are generally starch base adhesives. The earliest corrugating board adhesive patent, U.S. Pat. No. 2,102,937, discloses an adhesive composition comprising gelatinized starch, ungelatinized starch, alkali and borax in a water base. This adhesive has formed the basis for the manufacture of essentially all corrugated paper board to date. This adhesive is a carrier adhesive, wherein a portion of the starch forms a carrier gelatinized phase and the balance of the starch forms an ungelatinized phase. Under conditions of heat and pressure the ungelatinized starch rapidly gelatinizes, absorbs water and causes a rapid increase in viscosity and adhesivity of the starch-adhesive composition. Borax increases the level of viscosity as well during final gelatinization.
The use of additives in starch based corrugating board and other types of adhesives has been the subject of substantial research in recent years. One useful adhesive additive is various polymers of polyvinyl alcohol. On the whole, the use of soluble polyvinyl alcohol polymers has been avoided since soluble polyvinyl alcohol has been believed to interact with borax in the presence of alkali and starch to form useless gels. In order to avoid these difficulties a variety of patents have taught that by varying such parameters as the pH of the adhesive of the solubility of the polyvinyl alcohol, a stable polyvinyl alcohol containing adhesive can be formed. For example, Nickerson, U.S. Pat. No. 3,720,633 teaches that at acid pH's polyvinyl alcohols can be used in adhesive compositions in the presence of polyhydroxy compounds without the formation of substantial gels. Czerwin, U.S. Pat. No. 4,094,718 and Imoto, Japanese Pat. Publication No. 45-19600 teach that polyvinyl alcohol can be used in corrugating board adhesives containing starch, alkali and borax only if the polyvinyl alcohol compositions are substantially insoluble and prevented from reacting with the adhesive components until the adhesive is applied to the corrugating medium and heated past the gel temperature of the starch. Not only does Nickerson indicate that the use of soluble polyvinyl alcohols can result in the formation of intractable gels but Czerwin treats the insoluble polyvinyl alcohol in order to remove soluble polyvinyl alcohol that could solubilize and interact with the adhesive components and form a gel. However Czerwin teaches that some soluble polyvinyl alcohol tends to be present in the adhesive. In Krankkala, U.S. Pat. No. 4,600,739, certain types of partially hydrolyzed soluble polyvinyl alcohol composition below a certain concentration were found to be compatible with corrugated board adhesives containing starch, alkali and borax. The addition of the controlled amounts of a specific soluble polyvinyl alcohol was found to substantially increase green bond strength and rate of manufacture of the corrugated board. The amounts of the soluble polyvinyl alcohol that can be used is limited by the fact that at higher concentrations than claimed the soluble polyvinyl alcohols tend to form the gels recognized in Czerwin and Imoto. Further, soluble. polyvinyl alcohol tends not to provide any substantial water resistance to the glue line since the polyvinyl alcohol composition is stable.
The speed at which corrugating machines can be run is limited by the rapidity of viscosity increase in the bond line between the liner and corrugated medium. As the viscosity of the adhesive rises under conditions of heat and pressure in the corrugating machine, a "green bond" is formed which holds the components together until the adhesive system forms the final heat-cured corrugated bond. The green bond relies solely on the viscosity of the product to maintain the integrity of the product during curing.
Conventional starch based adhesives rely on the response of borax and ungelatinized starch under conditions of heat and pressure to increase the viscosity of the adhesive during green bond formation. More recent corrugating adhesives containing the additionally insoluble polyvinyl alcohol utilize the increase in viscosity that results as the polyvinyl alcohol is solubilized at high temperature. However, both the conventional and the solid polyvinyl alcohol containing conventional adhesives increase viscosity at a rate that leaves substantial room for improvement. If the rate at which viscosity increase occurs in the bond line can be increased, then the corrugating machines can be run at a substantial increase in speed resulting in an increase in productivity.
Accordingly a substantial need exists for a corrugating adhesive having a faster set or a substantial increase in the rate of viscosity development or in the rate of green bond formation during the manufacture of corrugated board.